Produce consistent moderator checking results prior to function call.

Fixes the inconsistency that was introduced in
d0cf68c32d9a266bc77cf44ed04faf9ee0df8b19 between the following two calls:

    list([x.subreddit for x in r.get_mod_mail('foo', limit=10)])

    list([x.subreddit for x in r.get_mod_mail(subreddit='foo', limit=10)])

In the latter, PRAW would not check that the user was a moderator prior to the
function call because `subreddit` would always be set to `mod` by default.